Simultaneous two-color snapshot view on ultrafast charge and spin dynamics in a Fe-Cu-Ni tri-layer

Abstract

Ultrafast phenomena on a femtosecond timescale are commonly examined by pump-probe experiments. This implies multiple measurements, where the sample under investigation is pumped with a short light pulse and then probed with a second pulse at various time delays to follow its dynamics. Recently, the principle of streaking extreme ultraviolet (XUV) pulses in the temporal domain has enabled recording the dynamics of a system within a single pulse. However, separate pump-probe experiments at different absorption edges still lack a unified timing, when comparing the dynamics in complex systems. Here, we report on an experiment using a dedicated optical element and the two-color emission of the FERMI XUV free-electron laser to follow the charge and spin dynamics in composite materials at two distinct absorption edges, simultaneously. The sample, consisting of ferromagnetic Fe and Ni layers, separated by a Cu layer, is pumped by an infrared laser and probed by a two-color XUV pulse with photon energies tuned to the M-shell resonances of these two transition metals. The experimental geometry intrinsically avoids any timing uncertainty between the two elements and unambiguously reveals an approximately 100 fs delay of the magnetic response with respect to the electronic excitation for both Fe and Ni. This delay shows that the electronic and spin degrees of freedom are decoupled during the demagnetization process. We furthermore observe that the electronic dynamics of Ni and Fe show pronounced differences when probed at their resonance, while the demagnetization dynamics are similar. These observations underline the importance of simultaneous investigation of the temporal response of both charge and spin in multi-component materials. In a more general scenario, the experimental approach can be extended to continuous energy ranges, promising the development of jitter-free transient absorption spectroscopy in the XUV and soft X-ray regimes.

Time-Resolved XUV Absorption Spectroscopy and Magnetic Circular Dichroism at the Ni M2,3-Edges

“…In addition, Figure 3 shows that transient absorption changes for different applied fields do not systematically start at t = 0: the same dynamics are observed for both field directions during the first tens of fs after optical excitation for E = 64.6, 65.7, and 66.3 eV (highlighted grey regions). At first glance, this is in line with the recent observation of a delay between ultrafast electronic and magnetic dynamics [30,34]. For E = 65.7 eV, a closer look at the first 100 fs reveals a crossing between H + and H − curves.…”

“…Furthermore, their calculations suggested that, in addition to the pure absorption spectra, the MCD should also be affected on sub-picosecond timescales. While subsequent experiments remained limited to the L-edges of transition metals [17], very recent work by Yao et al [34] and Rösner et al [30] unveiled that similar effects can also be observed at the M-edges of Fe, Co, and Ni. Furthermore, both studies demonstrated that, depending on the probing energy, a delay between the magnetic and pure charge dynamics (up to 100 fs) can exist.…”

“…Indeed, performing pure absorption measurements at X-ray free electron lasers (FELs) has been challenging, due to intrinsic fluctuations of the photon parameters and the lack of a reliable and linear incident intensity monitor [25]. However, motivated by time-resolved absorption experiments performed at femtoslicing facilities [17,22], as well as time-and spin-resolved photoelectron spectroscopy at high harmonic generation (HHG) sources [26,27], new technical advances were recently made to probe X-ray absorption dynamics using FEL [25,[28][29][30][31] and HHG [32,33], thereby laying the ground for a systematic analysis of charge and spin dynamics [30,34].…”

Multifocus off-axis zone plates for x-ray free-electron laser experiments

“…The benefit for XFEL science has already been proven in two user experiments. In an experiment at the DiProI beamline at FERMI, the two-spot scheme using a superimposed grating of duty cycle of d = 0.5 or shift of s = 0.5 was used in a time-streaking experiment with two incident energies [34]. The same approach enabled the first user experiment at the SCS beamline at the European XFEL.…”

A novel free-electron laser single-pulse Wollaston polarimeter for magneto-dynamical studies

“… 9 Several important experimental techniques had being developed within the FERMI laboratories, such as EUV transient-grating (TG) and EUV magnetic-TG, 10,11 simultaneous time, and two-color spectroscopy. 12,13 Motivated by the rapidly growing field of the so-called femtomagnetism, 14 we have designed, built, and commissioned a novel and effective polarimeter for ultrafast magneto-optical experiments in the EUV photon energy range.…”